1. Field of the Invention
The present invention relates to a production process for a vinyl-based polymer, and more specifically to a production process for polymerizing a vinyl-based monomer by a radical reaction with a characteristic method of adding a reaction inhibitor.
2. Description of the Prior Art
In conventional radical polymerization reactions of vinyl-based monomers, reaction inhibitors (also known as reaction retardants or reaction suppressants) have typically been added to the polymerization mixture. Examples of known reaction inhibitors include phenol-based compounds, sulfur compounds, N-oxide compounds, phosphorus compounds, and unsaturated hydrocarbon compounds. Specific examples of the phenol-based compounds include 2,2-di-(4xe2x80x2-hydroxyphenyl)propane, hydroquinone, p-methoxyphenol, t-butylhydroxyanisole, n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl) propionate, t-butylhydroquinone, 2,5-di-t-butylhydroquinone, 4,4xe2x80x2-butylidenebis-(3-methyl-6-t-butyl)phenol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2xe2x80x2-methylene-bis-(4-ethyl-6-t-butyl)phenol, triethyleneglycol-bis [3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], pentaerythrityl-tetrakis [3-(3,5-di-t-butyl-4-hydroxyphenol)propionate], t-butylcatechol, 4,4xe2x80x2-thiobis-(6-t-butyl)-m-cresol, and tocopherol.
The addition of these reaction inhibitors is performed for a variety of reasons, and they may be added to the polymerization mixture prior to commencement of the polymerization reaction in order to reduce the occurrence of fish eyes within the product polymer (Japanese Laid-open publication (kokai) No. 48-49990 (JP48-49990A), Japanese Post-Examination Publication (kokoku) No. 60-50366 (JP60-50366B)), added during the polymerization reaction in order to suppress heat kick, or alternatively added at the completion of the polymerization reaction in order to prevent postpolymerization and prevent deterioration in the anti-initial discoloration property resulting from heat history (U.S. Pat. No. 3,642,756, Japanese Laid-open publication (kokai) No. 57-185302 (JP57-185302A), and Japanese Laid-open publication (kokai) No. 62-503 (JP62-503A)). Furthermore, such reaction inhibitors are also used for halting rapid reactions which occur during abnormal reactions.
Amongst the above reaction inhibitors, 2,2-di-(4xe2x80x2-hydroxyphenyl)propane displays particularly suitable reaction inhibiting properties, enables the production of high quality polymers, and also produces very little adhesion of scale to the polymerization vessel, and has consequently been widely used at the completion of polymerization reactions. However, 2,2-di-(4xe2x80x2-hydroxyphenyl)propane is a solid at room temperature, and unless the material is dissolved in an organic solvent such as methanol prior to use, then the operation of supplying the reaction inhibitor to the reaction vessel via a supply pipe is problematic. However, because this method requires the use of an organic solvent, not only are there associated physical dangers for the operators exposed to the fumes, but these fumes also become a source of potential environmental pollution.
Consequently, in Japanese Post-Examination publication (kokoku) No. 7-113041 (JP7-113041B), a reaction inhibitor represented by a general formula (1) shown below was disclosed as a potential alternative to 2,2-di-(4xe2x80x2-hydroxyphenyl)propane. 
(wherein, R represents an alkyl group of 3 or more carbon atoms). Specifically, the compound in which the R group is a sec-butyl group, namely, 2,6-di-t-butyl-4-sec-butylphenol is currently used. This compound exists as a liquid in the supply tank and piping when the external air temperature is between 20 and 25xc2x0 C., namely room temperature, and so is able to be supplied to the polymerization vessel via the supply piping without requiring the use of a solvent. Furthermore, 2,4-dimethyl-6-(1-methylpentadecyl)phenol and 2,6-di-tert-butyl-4-nonylphenol are also liquids at room temperature, and can also be supplied to a polymerization vessel without the use of a solvent.
However, these type of reaction inhibitors also suffer problems if the external air temperature drops below normal room temperatures. For example, if 2,6-di-tert-butyl-4-sec-butylphenol is used, then because the melting point is from 18 to 20xc2x0 C., in cases in which the external temperature is low, namely 10xc2x0 C. or lower, the compound enters a supercooled state, and as a result, the reaction inhibitor is more likely to solidify, thereby blocking the supply piping. In a worst possible case, the reaction inhibitor may solidify inside the storage tank, making the supply operation itself impossible. In addition, in the case of other reaction inhibitors with lower melting points, such as 2,6-di-tert-butyl-4-nonylphenol, the viscosity at 20xc2x0 C. is approximately 400 mPaxc2x7s, and this viscosity exceeds 1000 mPaxc2x7s under cooling, which can make the supply operation extremely difficult. Consequently, an organic solvent such as methanol must be used to dissolve the reaction inhibitor, and so these types of reaction inhibitors do not completely resolve the aforementioned problems of physical danger for the operators and environmental pollution.
Furthermore, methods in which a dispersion such as an emulsion or a suspension is first prepared by using a dispersant such as an emulsifier or a suspension agent to disperse the above type of reaction inhibitor in an aqueous medium, and this dispersion is then added to the polymerization mixture can also be effective, although the viscosity of the dispersion may increase markedly, making the operation of supplying the dispersion through the supply piping to the polymerization vessel extremely difficult. Moreover, if the external temperature is less than 0C, then there is a danger of the dispersion solidifying.
In addition, in those cases where cold temperatures result in the reaction inhibitor solidifying and blocking the supply piping, a device for heating the blocked sections to melt the solidified reaction inhibitor and prevent blocking of the piping can be installed, although the heating operation is complex, and not only does melting the reaction inhibitor take considerable time, but there is also some danger associated with the heating process.
The present invention takes the above factors in consideration, with an object of providing a production process for a vinyl-based polymer in which a reaction inhibitor can be added to a polymerization mixture without the use of an organic solvent, even at low temperatures, and problems such as the solidification of the reaction inhibitor inside the supply tank or piping, and subsequent blocking of the piping do not occur.
As a result of intensive investigations aimed at resolving the above problems, the inventors of the present invention discovered that by heating the reaction inhibitor supply tank and the reaction inhibitor supply piping, and maintaining the reaction inhibitor, with a melting point of no more than 40xc2x0 C., in a liquid form with a viscosity below a specified level, problems such as the blocking of the supply piping due to solidification of the reaction inhibitor could be resolved, and the inventors were consequently able to complete the present invention.
In other words, the present invention provides a production process for a vinyl-based polymer comprising:
a step for polymerizing a vinyl-based monomer by a radical reaction within an aqueous medium in a polymerization vessel; and
a step for supplying a reaction inhibitor with a melting point of no more than 40xc2x0 C. from a reaction inhibitor supply tank to the polymerization vessel via a reaction inhibitor supply pipe;
wherein the reaction inhibitor supply tank and the reaction inhibitor supply pipe are heated, and the reaction inhibitor is added to the polymerization vessel in a liquid state with a viscosity of no more than 200 mPaxc2x7s without using an organic solvent.
As follows is a more detailed description of the present invention.
In the present invention, a reaction inhibitor with a melting point of no more than 40xc2x0 C. is maintained at a temperature above the melting point by heating the reaction inhibitor supply tank and the supply piping, and the reaction inhibitor is added to the polymerization mixture in a liquid state with a viscosity of no more than 200 mPaxc2x7s, and preferably no more than 50 mPaxc2x7s. If the viscosity of the reaction inhibitor exceeds 200 mPaxc2x7s, then not only is the reaction inhibitor more likely to adhere to the reaction inhibitor supply piping, making the supply of an accurate quantity of the inhibitor difficult, but when heating of the piping is halted and the piping cools, there is a danger of this adhered product solidifying and blocking the supply piping.
In cases in which the reaction inhibitor is in a supercooled state, namely a state in which the reaction inhibitor remains in a liquid state despite being cooled below melting point, the reaction inhibitor is far more likely to solidify. For example, if the reaction inhibitor is 2,6-di-tert-butyl-4-sec-butylphenol, which has a melting point of 18 to 20xc2x0 C., then at external air temperatures below 20xc2x0 C., solidification occurs. However, if the present invention is employed and the reaction inhibitor is added to the polymerization vessel in a liquid state with a viscosity maintained at no more than 200 mPaxc2x7s, then even if the external air temperature is low, the reaction inhibitor inside the supply tank and the supply piping is maintained at a temperature above the melting point. In addition, because the viscosity of the reaction inhibitor is low, the addition operation becomes a simpler operation. Accordingly, the reaction inhibitor can be added to the polymerization mixture without the use of an organic solvent such as methanol or toluene, and without blocking the reaction inhibitor supply piping between the reaction inhibitor supply tank and the polymerization vessel, even at low temperatures.
Heating of the reaction inhibitor supply tank and the reaction inhibitor supply piping is performed using a method such as that described below. There are no particular restrictions on the heating method employed, although typically heating is performed by passing hot water or steam through a jacket fitted to the polymerization vessel and the piping.
Heating of the supply tank must be performed when the external air temperature falls below the melting point of the reaction inhibitor, and particularly if the external air temperature falls to a temperature more than 10xc2x0 C. below the melting point, hot water must be passed through the jacket to maintain the temperature of the reaction inhibitor inside the supply tank and the supply piping at a value within a range from Txc2x0 C. to 60xc2x0 C. (where Txc2x0 C. represents the melting point of the reaction inhibitor), and preferably from (T+5)xc2x0 C. to 60xc2x0 C., and even more preferably from (T+5)xc2x0 C. to (T+20)xc2x0 C. If the temperature of the reaction inhibitor inside the supply tank is lower than the melting point, then there is a danger of the reaction inhibitor solidifying inside the tank. In contrast, if the temperature of the reaction inhibitor inside the supply tank exceeds 60xc2x0 C., then there is a danger that long term storage of the reaction inhibitor will result in deterioration and coloration of the inhibitor due to oxidation, causing coloration of the product polymer.
The reaction inhibitor supply tank and supply piping should preferably be maintained at a temperature within a range from 40 to 170xc2x0 C. using, e.g., a jacket provided on the outside thereof, and even more preferably from 70 to 140xc2x0 C. There are no particular restrictions on the heating method used to ensure maintenance of the temperature at this level, and hot water or steam heated to a temperature between 40 and 170xc2x0 C. can be used. The supply tank region should preferably be heated using hot water between 40 and 80xc2x0 C., and the supply piping heated using steam at a temperature between 100 and 150xc2x0 C. If the heating temperature is less than 40xc2x0 C., then the solidification prevention effect on the reaction inhibitor is inadequate. In contrast, if the heating temperature exceeds 170xc2x0 C., then the reaction inhibitor becomes susceptible to oxidation, and there is danger of anti-oxidant coloring appearing, resulting in coloring of the product polymer.
Moreover, the heating should preferably be performed on the entire region from the reaction inhibitor supply tank through to the polymerization vessel, including the reaction inhibitor meter and the reaction inhibitor supply piping. If sections of the supply piping are not heated, then these sections are cooled by the external air, meaning not only will the viscosity of the reaction inhibitor increase, making the supply operation more difficult, but the danger arises of the reaction inhibitor falling to a temperature below the melting point, and solidifying and blocking the supply piping.
In the present invention, a reaction inhibitor with a melting point of no more than 40xc2x0 C. is used. Examples of preferred reaction inhibitors include those represented by a general formula (1) shown below, 
(wherein, R represents an alkyl group of 3 or more carbon atoms), as well as 2,4-dimethyl-6-(1-methylpentadecyl)phenol (melting point 15xc2x0 C.).
Specific examples of the reaction inhibitor represented by the general formula (1) include 2,6-di-tert-butyl-4-sec-butylphenol (melting point 20xc2x0 C.), 2,6-di-tert-butyl-4-nonylphenol (melting point xe2x88x9240xc2x0 C.).
Examples of particularly preferred reaction inhibitors with melting points of no more than 40xc2x0 C. include 2,6-di-tert-butyl-4-sec-butylphenol, 2,6-di-tert-butyl-4-nonylphenol, and 2,4-dimethyl-6-(1-methylpentadecyl)phenol (melting point 15xc2x0 C.).
Depending on the effect desired, the reaction inhibitor is added to the polymerization mixture during at least one of three stages, namely, prior to commencement of the polymerization reaction, during the polymerization reaction, or at the completion of the polymerization reaction. The quantity of the reaction inhibitor added is typically within a range from 0.0005 to 0.5 parts by weight per 100 parts by weight of the vinyl-based monomer. Specifically, in those cases in which the reaction inhibitor is added prior to commencement of the polymerization reaction in order to reduce the occurrence of fish eyes within the product polymer, the quantity of the reaction inhibitor should preferably be within a range from 0.0005 to 0.005 parts by weight per 100 parts by weight of the vinyl-based monomer. In such cases, heating the aqueous medium prior to the addition is an effective method. In those cases in which the reaction inhibitor is added at the completion of the polymerization reaction once a predetermined rate of polymerization conversion has been reached, in order to prevent any subsequent postpolymerization, the quantity of the reaction inhibitor should preferably be within a range from 0.005 to 0.05 parts by weight per 100 parts by weight of the vinyl-based monomer. In addition, in those cases in which a reaction inhibitor of the present invention is added to the polymerization mixture in order to immediately and completely halt the suspension polymerization reaction, the quantity of the reaction inhibitor should preferably be within a range from 0.2 to 0.5 parts by weight per 100 parts by weight of the added vinyl-based monomer.
In a production process for a vinyl-based polymer according to the present invention, suitable examples of the vinyl-based monomer include halogenated vinyl or halogenated vinylidene compounds such as vinyl chloride, vinyl bromide and vinylidene chloride; polymerizable olefin-based monomers with at least one terminal CH2xe2x95x90C less than  group including acrylate esters such as methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate and cyanoethyl acrylate; vinyl acetate; methacrylate esters such as methyl methacrylate and butyl methacrylate; styrene and styrene derivatives such as xcex1-methylstyrene, vinyltoluene and chlorostyrene; vinylnaphthalene; diolefins such as butadiene, isoprene and chloroprene; as well as mixtures of the above monomers with other copolymerizable olefin monomers; and other known polymerizable olefin monomers.
A production process for a vinyl-based polymer according to the present invention can be applied to any type of vinyl-based monomer radical polymerization reaction, regardless of form, including suspension polymerization, emulsion polymerization, bulk polymerization and microsuspension polymerization.
As follows is a description of a vinyl-based polymer production process of the present invention, in the case of a suspension polymerization.
Suspension polymerization is carried out in an aqueous medium, in the presence of a known polymerization initiator and a dispersant, and typically at a temperature between 0 and 100xc2x0 C., with temperatures from 30 to 70xc2x0 C. being particularly preferred. There are no particular restrictions on the dispersant and the polymerization initiator used, and compounds used in conventional vinyl-based monomer polymerization reactions are suitable. Specific examples of the dispersant include water soluble cellulose ether compounds such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropylmethyl cellulose, water soluble or oil soluble partially saponified polyvinyl alcohols, water soluble polymers such as acrylic acid polymers and gelatin, oil soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, and block copolymers of ethylene oxide and propylene oxide, and water soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate and sodium laurate, and these dispersants may be used singularly, or in combinations of two or more different dispersants. There are no particular restrictions on the amount of dispersant added, although typically from 0.01 to 5 parts by weight of the dispersant is used per 100 parts by weight of the monomer.
Examples of the polymerization initiator include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate and diethoxyethyl peroxydicarbonate, perester compounds such as t-butyl peroxyneodecanoate, t-butyl peroxypivalate, t-hexyl peroxypivalate, xcex1-cumyl peroxyneodecanoate and 2,4,4-trimethylpentyl-2-peroxy-2-neodecanoate, peroxides such as acetyl cyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxy acetate, 3,5,5-trimethylhexanoyl peroxide and isobutyryl peroxide, and azo compounds such as azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy)-2,4-dimethylvaleronitrile, as well as ammonium persulfate and hydrogen peroxide and the like, and these polymerization initiators may be used singularly, or in combinations of two or more different initiators. There are no particular restrictions on the amount of polymerization initiator added, although typically from 0.01 to 1 part by weight of the polymerization initiator is used per 100 parts by weight of the monomer.
In term of other conditions associated with the suspension polymerization, there are no particular restrictions on factors such as the method of supplying the aqueous medium to the polymerization vessel and performing degassing, the method of supplying the vinyl monomer, other comonomers if required, the dispersant and the polymerization initiator, or the relative proportions of the above constituents, and typical conditions are suitable. Moreover, where necessary, other additives typically used in the polymerization of vinyl-based monomers such as polymerization degree regulators, chain transfer agents, pH regulating agents, gelation modifiers, antistatic agents, antioxidants and scale adhesion prevention agents may also be added to the polymerization mixture.